A conventional object detecting apparatus is disclosed in a Japanese Patent Document JP-A-2002-031685. The apparatus comprises a light emitting device that emits a laser beam, a polygonal mirror in a shape of a six-sided horizontally-sliced pyramid that rotates to reflect the laser beam, and a light receiving device that picks up reflection of the laser beam by an object. The apparatus projects the laser beam reflected by the polygonal mirror toward the front of the vehicle. The laser beam reflected by the polygonal mirror is so controlled that it scans a predetermined area of space in front of the subject vehicle. Reflection of the laser beam from, for example, a preceding vehicle is then picked up by the light receiving device and gives a measurement of distance to the preceding vehicle.
The intensity of the laser beam used in this type of object detecting apparatus has to be a certain level or higher in order to measure a long range of distance. On the other hand, a person's eyes may be hypersensitive upon direct exposure to this intensity of laser beam at a very short distance.
Therefore, the object detecting apparatus changes the intensity of the laser beam according to the conditions of the subject vehicle. For example, an object detecting apparatus used in a preceding vehicle tracking system that controls the subject vehicle to follow a preceding vehicle at a predetermined distance changes the intensity of the laser beam on condition that the subject vehicle, and the preceding vehicle, is either running or stopping. That is, the intensity of the laser beam is lessened below a certain level in order to maintain a certain range of measurement with an assured safety to the eyes when the subject vehicle is stopping, compared to the intensity being used by the subject vehicle when it is running. This is because the laser beam of the apparatus will not be directed to the eyes when the subject vehicle is running, while it may be directed to the eyes when the subject vehicle is stopping.
The above-described situation explains the necessity of intensity control of the laser beam used in the object detecting apparatus. This situation necessitates a precise control of the intensity of the laser beam.
Typical characteristics of laser diodes, as shown in a diagram in FIG. 4, also support the necessity of the intensity control. Laser diodes used to generate the laser beam are low in fidelity in terms of Current (represented by “I” in the diagram)—Luminosity characteristics (I-L characteristics hereinafter) when luminosity (luminous intensity) of the laser beam is low. The difference of laser beam intensity against a certain electric current can be seen, for example, as a difference between the diversified characteristics curves of MAX, TYP, and MIN in FIG. 4. The I-L characteristics in this diagram differ in a wide range (vertically spread) when an input current is specified, or a certain target luminous intensity falls on a wide range (horizontally spread) of input current value depending on the I-L characteristics of each laser diode.
This leads to a problem that the intensity of the laser beam does not fit in a predetermined range even when an input current is determined based on the I-L characteristics curve because of the diversified I-L characteristics of each laser diode.
The I-L characteristics of the laser diode are also influenced by the temperature of the diode. That is, a certain value of electric current input applied to a laser diode does not always generate the same laser beam intensity. This is another cause of the same problem that the intensity of the laser beam does not fit in a predetermined range.